General process of heat treatment

Heat treatment is a kind of comprehensive technological process that heat, heat and cool the material in a certain medium, and control its performance by changing the surface or internal structure of the material.

Heat treatment is one of the important processes in mechanical manufacturing. Compared with other processes, heat treatment generally does not change the shape and overall chemical composition of the workpiece, but endows or improves the performance of the workpiece by changing the microstructure inside the workpiece or the chemical composition on the surface of the workpiece.

Heat treatment process generally includes three processes: heating, heat preservation and cooling, sometimes only heating and cooling. These processes are interlinked and uninterrupted.

Heating is one of the important processes of heat treatment. There are many heating methods for metal heat treatment, the first one is to use charcoal and coal as heat source, and then use liquid and gas fuel. The application of electricity makes heating easy to control and no environmental pollution. These heat sources can be used to heat directly, or indirectly through molten salt or metal, or even floating particles.

Heating temperature is one of the important process parameters of heat treatment process. The selection and control of heating temperature is the main problem to ensure the quality of heat treatment. The heating temperature varies with the metal material being treated and the purpose of heat treatment, but it is generally heated above the phase change temperature to obtain high temperature structure. In addition, the transformation needs a certain time, so when the surface of the metal workpiece reaches the required heating temperature, it must be kept at this temperature for a certain time, so that the internal and external temperatures are the same, and the microstructure is completely transformed, which is called the holding time. When high energy density heating and surface heat treatment are used, the heating speed is extremely fast, and there is generally no heat preservation time, while the heat preservation time of chemical heat treatment is often longer.

Cooling is also an indispensable step in the process of heat treatment. The cooling method varies with the process, mainly controlling the cooling speed. Generally speaking, the cooling speed of screw conveyor is the slowest in annealing, the cooling speed of normalizing is faster, and the cooling speed of quenching is faster. But there are different requirements due to different kinds of steel. For example, air hardening steel can be quenched at the same cooling rate as normalizing.

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